But how would going one extra stop and then detraining avoid any
hold up? It would just mean the hold up would be delayed by a couple of
minutes.

Why do I think someone doesn't know the track layout at East Finchley?

Hey thats why I was asking! The basic answer would have to be, there
is a reason. Whether its obvious to punters or not, LU aren't going to
cause chaos just for fun.

Like its been said, it was quite possibly a defective train, and in
such instances it would be unsafe to keep the train in customer service.